Considerable research efforts have been made to develop high strength aluminum which would satisfy the demands of advanced design in aircraft, automotive and electrical industries. Aluminum-lithium alloys are amongst those under consideration because of the potential that the addition of lithium to aluminum offers for improving properties of aluminum with respect to density and elastic modulus. However, the improvement of one or even two properties does not mean the alloy will be useful for certain advanced design applications. Rather, for the alloy to be useful, it must meet all minimum target property requirements. Such properties as density, strength, ductility, toughness, fatigue and corrosion resistance, are among the properties considered.
Heretofore, many aluminum-lithium alloy systems prepared by ingot metallurgy techniques have been studied. Also, various aluminum-lithium, aluminum magnesium and aluminum-copper-magnesium systems which have been prepared by mechanical alloying techniques, have been studied. However, none have been entirely satisfactory for certain applications which require low density, high strength, corrosion resistance, fracture toughness and good ductility. Fracture toughness is a measure of the ability of a material to tolerate a flaw, i.e., a crack, without fracturing. The terms "toughness" and "fracture toughness" are used herein interchangeably.
The mechanical alloying technique has been disclosed, for example, in U.S. Pat. Nos. 3,591,362; 3,740,210; and 3,816,080. Mechanical alloying, as described in the aforesaid patents, is a method for producing composite metal powders with a controlled, uniform fine microstructure. It occurs by the fracturing and rewelding of a mixture of powder particles during high energy impact milling in a controlled environment, e.g. in an attritor grinding mill, in the presence of a process control agent. In the process, dispersoid materials such as, for example, the naturally occurring oxide on the surface of powder particles are incorporated into the interior of the composite powder particles and homogeneously dispersed therethrough. In a similar fashion, metallic alloy ingredients are also finely distributed within the powder particles. The powders produced by mechanical alloying are subsequently consolidated into bulk forms by various methods such as hot compaction followed by extrusion, rolling or forging.
A major problem with many conventional aluminum-lithium alloys is that when they meet requirements of density and strength, they are not sufficiently ductile or tough to be useful. In accordance with the present invention, alloys are provided which have ductility as well as a combination of low density, high strength and fracture toughness.